What is attribution window?
An attribution window is the length of time a platform will credit a conversion back to an ad interaction, for example Meta's default 7-day-click, 1-day-view. Change the window and the same underlying conversions redistribute across campaigns differently, which means a reported performance swing after a window change is a measurement artifact, not a real change in results.
Every ad platform has to draw a line somewhere: how many days after someone clicks or sees an ad does a purchase still count as caused by that ad? That line is the attribution window, and platforms set different defaults (Meta's has moved between 1-day and 7-day click windows over time; Google Ads uses a data-driven or last-click model with its own lookback).
The reason this matters more than it sounds: attribution windows aren't just a reporting preference, they change which campaigns get credit for which conversions. A brand campaign with a long consideration cycle looks worse under a short window (its conversions happen too late to count) and a retargeting campaign looks artificially strong under a long window (it's catching credit for demand a brand campaign actually generated upstream).
The practical failure mode: a platform changes its default window, or an advertiser changes it manually, and the reported numbers jump. Read that jump as a market win or loss and you'll make a real budget decision on a measurement artifact. The fix is simple but often skipped: never compare conversion counts across an attribution-window change without noting the change happened, and prefer platform-level trends (which usually stay on a consistent window) over cross-window comparisons.